yes, during transpiration if the leaves do this they are trying to conserve water in the plant its self...and this usually happens when the salt content of the media is too high...and this happened to me with my seedlings...like i said...they were fine til about 12" tall, then it started...and i never gave them any ferts before then either...just plain water...but when i checked the runoff of the healthy vs non-healthy, you seen the results i posted above....and this was purely from worm castings and azomite, and dolomite lime, mixed in
promix...
im going to guess that your not letting enough runoff out the bottom to test...that is why your only getting 300 ppm...if i water my plants when dry with plain water, then catch a runoff, it will be at about 500ppm...if i wait, and let the plant drink slower, until saturated, but with no runoff, then let her sit an hour or so to let all the salts in the soil/media to disolve int the water...then after about an hour ill come back, and pour about one liter of water, and about one liter of water will come out the bottom...ill test those and ppm will be about 850...i do this 2 more times, and the next is 1100, and the 3rd is 1800.....the 3rd one is the key, it tells me that the ppms are rising, not dropping, and that there is too much salts in the medium for the plant to thrive and transpire properly....salt content and heat are the 2 main reasons for the taco as you have...
i would like to reccomend you test runoff again ......this time water plant very slowly until its fully saturated and only a very very small amount of runoff stays at the bottom...remove runoff, now flood plants again to collect runoff..do this 3 times and test runoff each time....record the difference between each reading.....to get an accurate reading each time, make sure to get between 10-20% runoff each time